1. Field of Invention
This invention relates to molecular sieve adsorbents and more particularly to processes of use of a molecular sieve adsorbent blend comprising a zeolite and a highly dispersed attapulgite binder. This invention also relates to processes of use of molecular sieve adsorbent blends prepared by the process of blending a zeolite with a highly dispersed attapulgite binder.
2. Background Art
Zeolites are hydrated metal alumino silicates having the general formulaM2/nO:Al2O3:xSiO2:yH2Owhere M usually represents a metal of the alkali or alkaline earth group, n is the valence of the metal M, x varies from 2 to infinity, depending on the zeolite structure type and y designates the hydrated status of the zeolite. Most zeolites are three-dimensional crystals with a crystal size in the range of 0.1 to 30 μm. Heating these zeolites to high temperatures results in the loss of the water of hydration, leaving a crystalline structure with channels of molecular dimensions, offering a high surface area for the adsorption of inorganic or organic molecules. Adsorption of these molecules is limited by the size of the zeolite channels. The rate of adsorption is limited by the laws of diffusion.
One limitation on the utilization of these zeolite crystals is their extremely fine particle size. Large naturally-formed agglomerates of these crystals break apart easily. Because the pressure drop through the bed is prohibitively high, these zeolite crystals cannot be used in fixed beds for various dynamic applications, such as drying of natural gas, drying of air, separation of impurities from a gas stream, separation of liquid product streams and the like. Therefore, it is desirable to agglomerate these crystals with other materials to provide an agglomerate mass of the crystals which exhibits a reduced pressure drop.
To permit the utilization of these molecular sieve crystals, different types of clays are used as binders including attapulgite, palygorskite, kaolin, sepiolite, bentonite, montmorillonite and mixtures thereof. For example, U.S. Pat. No. 2,973,327 discloses the use of a number of different types of clays, including attapulgite, as a binder for molecular sieves. The clay content of the bonded molecular sieve can vary from as low as 1 percent to as high as 40 percent by weight, although the preferred range is from about 10 to about 25 percent by weight.
U.S. Pat. No. 3,219,590 discloses another molecular sieve blend comprising a kaolin-type clay and a lignosulfonate which functions as the binding agent.
Adsorbent materials comprising a type 5A zeolite molecular sieve and a kaolin clay binder, wherein the kaolin comprise from about 10 to about 40 percent of the composition, are disclosed in U.S. Pat. No. 5,001,098.
Molded bodies containing dealuminated zeolite Y and a binder material are disclosed in U.S. Pat. No. 5,316,993.
An adsorbent and/or catalyst blended with a binder system comprising a colloidal metal oxide, an oxide adsorbent and an acid are disclosed in U.S. Pat. No. 5,948,726.
An adsorbent for separating gases comprising a binder and a crystalline, low silica faujasite-type zeolite with a silica to alumina molar ratio of 1.9 to 2.1 is disclosed in EP 0 940 174 A2. This reference discloses the blending of a zeolite with a conventional, dense attapulgite clay binder useful for the separation of gases. The bulk density of the binder is not disclosed.
Another blend of a conventional, dense attapulgite clay binder with a zeolite is disclosed in U.S. Pat. No. 5,413,978. The bulk density of the attapulgite clay is from about 400 g/l to about 530 g/l.
An abrasion-resistant granular zeolite formed by blending a zeolite and a binder system is disclosed in U.S. Pat. No. 4,420,419. See also U.S. Pat. No. 5,292,360 which discloses an adsorbent for the purification of gases comprising a 5A zeolite molecular sieve and a kaolin clay binder.
One problem with conventionally formed zeolite blends is decreased diffusion. The larger the diameter of the formed zeolites, the slower the rate of diffusion of the molecules to be adsorbed. Particularly in the field of pressure swing adsorption, this effect is highly adverse to short cycle time and thus to productivity. Enhanced kinetic values or faster mass transfer rates can result in shorter cycle time and lower power consumption and thus higher adsorbent productivity.
It has been recognized that a reduction in the particle size of formed zeolites leads to shorter mass transfer zones and shorter cycle times. This is based on the assumption that the time needed for adsorbates to travel through the macropores of the adsorbents limits the cycle time, i.e. macropore diffusion is the rate limiting step in these processes. This problem can be improved by adding pore forming compounds to the zeolite clay blend before the forming step.
Accordingly it is an object of the invention to disclose a process for the preparation of molecular sieve adsorbents with enhanced diffusion rates.
It is a still further object of the invention to disclose a process for the production of a molecular sieve adsorbent blend which is especially useful in thermal swing adsorption (TSA) systems and in pressure swing adsorption (PSA) systems.
It is a still further object of the invention to disclose molecular sieve adsorbent blends which maintain their physical properties and diffusion capabilities even with reduced binder percentages.
It is a still further object of the invention to disclose a process for the production of a molecular sieve adsorbent blend utilizing highly dispersed attapulgite fibers.
It is a still further object of the invention to disclose a molecular sieve adsorbent blend comprising a zeolite powder and a highly dispersed attapulgite binder.
It is a still further object of the invention to disclose a process for drying a feed stream comprising passing the feed stream over a molecular sieve adsorbent blend comprising a zeolite and a highly dispersed attapulgite binder.
It is a still further object of the invention to disclose a process for the adsorption of carbon dioxide from an air stream comprising passing that air stream over a molecular sieve adsorbent blend comprising a zeolite powder and a highly dispersed attapulgite binder.
It is still further object of the invention to disclose a process for separation of components of a gaseous or liquid feed stream comprising passing that gaseous or liquid feed stream over a molecular sieve adsorbent blend comprising a zeolite powder and a highly dispersed attapulgite binder.
These and other objects are obtained by the process for production, the process for use and product of the invention disclosed herein.